How would i go about creating a "character design" script?For example, you chose a male or female body with the default clothing, and then as you progress in the game you will be able to changs their eyes, hair, coat, t-shirt etc

If the images are static (as in, no animation) there are several libraries out there that handle stuff like this. I don't know of any off the top of my head but they are often called "paper doll" or "human avatar" systems. If the characters are animated, it gets a bit more complicated then and no off-the-shelf library is going to meet that need. At that point it becomes more about trying to find a way to prevent having to animate all of the pieces of clothing, hair, etc. That is why in most 16-bit era RPGs, character's hair and shirt don't move when they walk so they just need the 3 directions of the hat and shirt art assets (you only need one left or right of course, since they can be reversed) and position them over the base avatar sprite.